select readfileIds ?writefileIds? ?exceptfileIds? ?timeout?
          This command allows an Extended Tcl program to wait  on
          zero  or  more files being ready for for reading, writ-
          ing, have an exceptional condition pending,  or  for  a
          timeout  period  to expire.  readFileIds, writeFileIds,
          exceptFileIds are each lists of  fileIds,  as  returned
          from  open, to query.  An empty list ({}) may be speci-
          fied if a category is not used.

          The files specified by the readFileIds list are checked
          to  see  if  data  is  available  for reading. The wri-
          teFileIds are checked if the specified files are  clear
          for  writing.   The exceptFileIds are checked to see if
          an exceptional condition has  occurred  (typically,  an
          error).   The write and exception checking is most use-
          ful on devices, however, the read checking is very use-
          ful  when communicating with multiple processes through
          pipes.  Select considers  data  pending  in  the  stdio
          input buffer for read files as being ready for reading,
          the files do.  not have to be unbuffered.

          Timeout is a floating point timeout value, in  seconds.
          If an empty list is supplied (or the parameter is omit-
          ted), then no timeout is set.  If the  value  is  zero,
          then  the  select  command  functions  as a poll of the
          files, returning immediately even if none are ready.

          If the timeout period expires with none  of  the  files
          becoming ready, then the command returns an empty list.
          Otherwise the command returns a list of three elements,
          each  of  those  elements is a list of the fileIds that
          are ready in the read, write and exception classes.  If
          none  are  ready  in a class, then that element will be
          the null list.  For example:

                  select {file3 file4 file5} {file6 file7} {} 10.5

          could return

                  {file3 file4} {file6} {}

          or perhaps

                  file3 {} {}

          On Windows 95/NT, only sockets can  be  used  with  the
          select  command.   Pipes,  as returned by the open com-
          mand, are not supported.

          This command is provided by Extended Tcl.